The only time I've truly thought that a Gem was the absolute best item to use on a Pokemon was when I used the following set in OU:
move 1: Shell Smash
move 2: Aqua Jet
move 3: Waterfall
move 4: Stone Edge
item: Rock Gem
ability: Sturdy
nature: Jolly
evs: 4 HP / 252 Atk / 252 Spe
Beautiful and simple set - use with Rain support, make sure the field is clear of hazards and boost on basically anything thanks to Sturdy. Waterfall becomes super-strong and +2 rain-boosted Aqua Jet picks off most priority users or Scarfers.
The Rock Gem is the final part of the puzzle - because most of his sweeping was done at 1HP a Life Orb would have obviously been ridiculous and other boosting items were too weak. I had also found that Stone Edge was only usually called upon once during an average sweep, with his boosted Water attacks usually being enough to take out most of a weakened team. But still, Stone Edge would always fall short against bulky waters and grass types and it just started to feel like he required way too much support to sweep effectively.
But then Rock Gem came along:
+2 Rock Gem Stone Edge vs 252/252+ Rotom-W = 97% - 114% (OHKO after SR)
+2 Rock Gem Stone Edge vs 252/220+ Celebi = 78% - 92% (33% chance to OHKO after SR)
+2 Rock Gem Stone Edge vs 252/252+ Politoed = 97% - 114% (OHKO after SR)
+2 Rock Gem Stone Edge vs 252/252+ Jellicent = 95% - 112% (OHKO after SR)
+2 Rock Gem Stone Edge vs 252/252+ Vaporeon 91% - 107% (OHKO after SR)
Probably one of my favourite sets in BW, it's inexplicable that it never made it into the analysis but bleh - try it out!